Formation of the yolk sac vascular system and its connection to the embryonic circulation is crucial for embryo survival in both mammals and birds. Most mice with mutations in genes involved in vascular development die because of a failure to establish this circulatory loop. Surprisingly,formation of yolk sac arteries and veins has not been well described in the recent literature. Using time-lapse video-microscopy, we have studied arterial-venous differentiation in the yolk sac of chick embryos. Immediately after the onset of perfusion, the yolk sac exhibits a posterior arterial and an anterior venous pole, which are connected to each other by cis-cis endothelial interactions. To form the paired and interlaced arterial-venous pattern characteristic of mature yolk sac vessels, small caliber vessels of the arterial domain are selectively disconnected from the growing arterial tree and subsequently reconnected to the venous system, implying that endothelial plasticity is needed to fashion normal growth of veins. Arterial-venous differentiation and patterning are controlled by hemodynamic forces, as shown by flow manipulation and in situ hybridization with arterial markers ephrinB2 and neuropilin 1, which show that expression of both mRNAs is not genetically determined but plastic and regulated by flow. In vivo application of ephrinB2 or EphB4 in the developing yolk sac failed to produce any morphological effects. By contrast, ephrinB2 and EphB4 application in the allantois of older embryos resulted in the rapid formation of arterial-venous shunts. In conclusion, we show that flow shapes the global patterning of the arterial tree and regulates the activation of the arterial markers ephrinB2 and neuropilin 1.

The adult vascular system is composed of an arterial, a venous and a lymphatic compartment. These different compartments respectively provide oxygen and nutrients to peripheral organs, remove carbon dioxide and waste products and maintain an immune barrier to defend the host against foreign organisms. Malfunctions of the vascular system represent a major cause of mortality and disease in developed countries. Understanding of the molecular mechanisms regulating vascular system development and maintenance is thus crucial for the design of therapies to cure vascular diseases. The molecules implicated in the control of physiological and pathological angiogenesis in the adult already function during embryonic development. Indeed, the survival of the embryo also critically depends on the establishment of a functional circulatory loop. Here we review our current knowledge about the emergence of endothelial precursor cells in the embryo, of their assembly into the primary vascular plexus and of the remodeling of this plexus into arteries and veins. We also focus on the molecular mechanisms controlling the development of arteries, veins and lymphatic vessels.

Neuropilin 2 is a receptor for class III semaphorins and for certain members of the vascular endothelial growth factor family. Targeted inactivation of the neuropilin 2 gene (Nrp2) has previously shown its role in neural development. We report that neuropilin 2 expression in the vascular system is restricted to veins and lymphatic vessels. Homozygous Nrp2 mutants show absence or severe reduction of small lymphatic vessels and capillaries during development. This correlated with a reduction of DNA synthesis in the lymphatic endothelial cells of the mutants. Arteries, veins and larger, collecting lymphatic vessels developed normally, suggesting that neuropilin 2 is selectively required for the formation of small lymphatic vessels and capillaries.